1. Field of the Invention
The present invention relates to an optical amplifier and an optical monitor circuit, which are mainly utilized for optical communications, and in particular, relates to an optical amplifier and an optical monitor circuit, which are provided with a monitoring function of detecting the power of noise light and the like generated when a signal light is amplified.
2. Description of the Related Art
An optical amplifier is one of key components for realizing the long distance and large capacity of an optical communication system. Optical amplifiers are classified into a laser amplifier using the stimulated emission from a population inversion medium and an amplifier based on a non-linear optical effect such as Raman scattering, Brillouin scattering or the like. Further, the laser amplifier includes a rare-earth element doped fiber amplifier and a semiconductor laser amplifier using a semiconductor amplification medium. The former is operated as an optical amplifier with the optical pumping, and the latter is operated as an optical amplifier with the injected current pumping. In these optical amplifiers, the rare-earth element doped optical fiber amplifier has a large advantage in terms of performance, for example, bit rate free, high gain, low noise, broadband, low coupling loss, low polarization dependence, high efficiency and the like. In the rare-earth element doped optical fiber amplifiers, an erbium (Er)-doped fiber amplifier (to be referred to as EDFA hereunder) is typical and is now in practical use in an optical fiber communication system. Such an optical amplifier is required to realize the higher performance while holding a simple optical circuit configuration, in order to achieve the improvement of the performance, the cost performance and the like of the optical communication system to which such an optical amplifier is applied. Therefore, demands for an optical amplifier realizing the higher performance are increased.
In a WDM optical communication system which repeatedly transmits a wavelength division multiplexed (WDM) light containing a plurality of optical signals of different wavelengths, it is desired that a wavelength characteristic of the signal light power is flat in order to satisfy a predetermined transmission characteristic. However, there is a problem in that the wavelength flatness of the signal light power on the reception side is deteriorated due to various factors, such as conditions of optical transmission path, the accumulation of gain wavelength characteristic (for example, tilt, ripple or the like) in an optical amplification repeating station using the rare-earth element doped optical fiber amplifier, the Raman amplifier and the like. Therefore, as one issue concerning an optical amplifier operation control, there is considered the establishment of technology for controlling an output wavelength characteristic of the optical amplification repeating station (first issue).
Further, in the optical amplifier, an amplified spontaneous emission (ASE) light being a noise component is generated with the optical amplification. This ASE light is generated over a broad wavelength band, although the optical power level thereof is significantly low compared with that of the signal light. Therefore, in the case of performing a control of the optical amplifier, such as an automatic level control or an automatic gain control, using a typical output monitoring technology in which an output light from the optical amplifier is branched to be received by a light receiver, since the optical power of the ASE light being the noise component is contained together with the signal light in the output light, an influence of the ASE light is reflected in the output monitoring result, thereby deteriorating the control accuracy of the optical amplifier. Moreover, an input shutdown function (function of detecting no-input of the input signal light power to shut off the pumping light power for the optical amplifier) of the optical amplifier arranged on the downstream is also deteriorated. Such a problem caused by the ASE light becomes serious, since a generation amount of the ASE light is significantly changed, particularly in a system or the like where the number of signal wavelengths is dynamically changed. Therefore, as another issue concerning the optical amplifier operation control, there is considered the establishment of technology for monitoring the ASE power generated with an optical amplifier correctly, and for correcting a control target value and a no-input detection threshold of the optical amplifier (second issue).
Moreover, in the optical amplifier, there is a problem in that, in a high population inversion state, energy concentrates in a wavelength range having a larger gain coefficient to cause an oscillating operation, and accordingly, the noise component is increased to deteriorate a transmission characteristic. This oscillation phenomenon of the optical amplifier includes a threshold according to a relationship between a gain of an optical amplification medium and reflection attenuation amounts on the input and output sides of the optical amplification medium, and therefore, the problem as described above is exposed due to factors, such as gain conditions of the optical amplifier, the deterioration of reflection attenuation amounts in optical components. To be specific, for example, in the case where the number of signals input to the optical amplifier is less or in the case where an isolation amount in an optical isolator on an optical path connected to the optical amplification medium is deteriorated, the oscillating operation (increase of noise component) becomes apparent. Therefore, as a further issue concerning the optical amplifier operation control, there is considered the establishment of control technology in which the output light level does not exceed an oscillation threshold in a previously known wavelength range of large gain coefficient (third issue).
In order to achieve the improvement of the performance and reliability of the WDM optical communication system, it is important to solve simultaneously the above described first to third issues concerning the optical amplifier operation control. To be specific, it is necessary to enable the monitoring with high accuracy of a wavelength characteristic of the WDM signal light power for the first issue, and to enable the monitoring with high accuracy of the power of the noise light, such as the ASE light and the like, for the second and third issues.
As a configuration of an optical power monitor in the conventional optical amplifier, as shown in FIG. 12 for example, there has been known a configuration in which an optical branching device 101 is inserted on a main signal system optical path through which a WDM light is propagated, and an optical spectrum analyzer (OSA) 102 is arranged on a branching port of the optical branching device 101, so that a measurement result of the optical spectrum monitored by the optical spectrum analyzer 102 is transmitted to a variable gain equalizer (VGEQ) 103 and the like, thereby performing the control. Further, there is also a configuration in which, instead of the optical spectrum analyzer 102, a wavelength separating device (for example, grating, optical filter or the like) and a light receiver are disposed on the branching port of the optical branching device 101, so that a light demultiplexed by the wavelength separating device is received by the light receiver to monitor the power thereof (Japanese Unexamined Patent Publication No. 2001-168841).
However, in the configuration of the conventional optical power monitor, it is necessary to newly add the optical branching device 101 and the expensive optical spectrum analyzer 102 or the wavelength separating device and the like to the optical amplifier. Therefore, there is a disadvantage of the complication and high cost of the optical circuit configuration.
Further, as exemplarily shown in the lower left of FIG. 12, the noise light, such as the ASE light or the like, whose optical power per unit micro-wavelength range is significantly low compared with that of the signal light, is branched at a branching ratio same as the signal light by the optical branching device 101. The branching ratio by the optical branching device 101 is set so that a ratio of the monitor light side becomes lower, since a decrease of the main signal light power needs to be suppressed as much as possible (for example, 95 to 99% on the main signal light side while 1 to 50% on the monitor light side, or the like). Therefore, the noise light contained in the monitor light becomes very little, so that the level of the noise light received by the optical spectrum analyzer 102 is low. Accordingly, there is also a problem in that the photosensitivity is poor and it becomes hard to monitor with desired accuracy the noise light power.
As one method for solving the problems in the conventional configuration as described above, there is considered that, for example, a typical value of the power of the noise light generated in the optical amplifier is previously obtained by an experiment, the simulation or the like, to perform the control of the optical amplifier using the obtained typical value. However, according to such a method, it is hard to estimate accurately the value of the power of the noise light generated in the optical amplifier, since the generation amount of the noise light is dynamically changed according to individual differences in components of the optical amplification medium, a change in environment (for example, temperature, humidity or the like), the number of wavelengths of the signal light contained in the WDM light or the like. Therefore, it becomes impossible to correct accurately the noise light component in the actually monitored optical output power. Consequently, the control accuracy of the optical amplifier becomes poor, thereby deteriorating the performance and reliability of the WDM optical communication system.